GENERAL OBSERVATIONS. 399 



nated also in the Lower Silurian and became extinct in the begin- 

 ning of the Devonian; Goniatites, which began in the Hamilton 

 period of the Devonian and disappeared with the Carboniferous 

 age. Many other instances are given in the table beyond. The 

 causes of such extinctions were connected with a higher principle 

 than that of mere physical catastrophe. 



The following table presents to the eye the history of many of the 

 genera, families, and tribes of Palaeozoic species, showing by means 

 of the narrow dark areas the time of their commencement ; the 

 time of their culmination (by the greatest breadth of the area) ; 

 and the time of their extinction in the course of the Palaeozoic 

 ages, or the fact of their continuing to survive in after-time. Thus, 

 opposite the word Polyps, the area commences near the beginning 

 of the Silurian, and increases through the Palaeozoic, and does not 

 terminate there, as they exist afterwards ; the Cyathophylloid Corals 

 begin with the Lower Silurian, have their maximum in the Devo- 

 nian, and terminate in the Carboniferous ; the Astrcea, Madrepora, 

 and Caryophyllia families of corals (which include most modern reef- 

 making species) do not commence until after the Palaeozoic, and 

 hence, as there are no Palaeozoic species, there is a dotted line op- 

 posite, without an area. At the top of the columns, P. Pd. stands 

 for Potsdam or Primordial Period ; and S., C, P., for Subcarboni- 

 ferous, Carboniferous, and Permian. 



4. Genera of the present time dating from the Palceozoic age. — The num- 

 ber of lines connecting the past with the present is considerably 

 increased in the Carboniferous age. These lines are, however, only 

 long-lived genera, not species. The following are those which 

 appear to be determined with a good degree of certainty : — 



Lingula, Discina, Crania, Nautilus, Pleurotomaria, Rhynchonella, Tere- 

 bratula, Ostrea, Avicula, Pinna, Pecten, Solemya, Leda, Nucula, Denta- 

 lium, Chiton. They are all Molluscan. The first five commenced in 

 the Lower Silurian. It is a remarkable fact that there are no Ra- 

 diate genera in this list. 



Besides the above, the genera Area, Lima, and Astarte have been 

 referred to the Palaeozoic ; but the species probably belong to other 

 genera. There are no genera of Articulates, unless it be the genus 

 Spirorbis, about which there is doubt. Some of the Palaeozoic genera 

 of Ostracoids have modern names, but there are no means of prov- 

 ing that they are identical in type with those of modern seas. 



There are modern genera of Rhizopods in the Palaeozoic, and pro- 

 bably also of Diatoms ; and the number of such genera among these 

 protozoan and protophyte forms will probably be greatly increased 

 when further investigated. 



